This invention relates to raster distortion correction circuits for television receivers.
To correct for raster distortion, such as side pincushion distortion, typically, the peak trace current in the horizontal deflection winding is varied in a parabolic manner at the vertical scanning rate by a correction circuit coupled to the deflection winding. Certain correction circuits provide this peak horizontal trace current variation by including a saturable reactor in series with the deflection winding, the inductance of which is varied in a parabolic manner at a vertical rate.
Other correction circuits, such as diode modulator circuits, provide the peak horizontal trace current variation by coupling an inductor and a modulating capacitor in series with the deflection winding and trace capacitor. The voltages across each of the inductances are modulated during the trace interval at a vertical parabolic rate, the voltages across the inductances being 180.degree. out-of-phase with each other. During each horizontal retrace interval, each inductance forms a separate resonant retrace circuit with respective retrace capacitors to achieve current reversal. The values of the retrace capacitors are selected to provide each resonant circuit with identical resonant retrace frequencies. To prevent the stray capacitance, leakage inductance, and load impedances of the flyback transformer from altering the above-described resonant frequencies, the junction of the deflection winding and diode modulator inductor is coupled through a retrace capacitor to a tap point on the flyback transformer primary.
Still other correction circuits provide the proper peak trace current variation by modulation during the horizontal retrace interval. Such circuits may include an inductance coupled to a controllable bidirectional switch and the horizontal deflection winding. During the second half of horizontal retrace, the retrace pulse energy in the retrace capacitor divides between the deflection winding and the inductance as a function of the on-off ratio of the switch during retrace. Varying the on-off ratio at a parabolic vertical rate corrects for side pincushion distortion. This on-off ratio variation will vary the effective inductance coupled to the retrace capacitor during horizontal retrace and change the resonant frequency of the retrace circuit causing retrace pulse time modulation. It is desirable when designing such a correction circuit to adjust such retrace pulse time modulation in a manner which will prevent the modulation from becoming excessive.